1. Field of the Invention
The present invention relates to a rubber-product-reinforcing steel cord (hereinafter, this may be simply referred to as a “cord”) used as a rubber-product-reinforcing material for an automotive tire, a conveyer belt, or the like, and a method for manufacturing the same.
2. Description of Related Art
As a single-layered rubber-product-reinforcing steel cord used as a rubber-product-reinforcing material for an automotive tire, a conveyer belt, or the like, conventionally, a cord is typically used that has a single-layered twist structure such as a 1×3 or a 1×5 structure, and that has a so-called closed-twist structure in which a plurality of wires are tightly intertwined in close contact with each other. However, in a cord having such a closed-twist structure, a closed space is formed at the center of the cord. When the cord is held between two rubber sheets and subjected to heat compression to form a composite sheet, rubber material does not penetrate the hollow portion at the center of the cord, and the composite has a structure in which the cord is merely enclosed by the rubber sheets. Thus, a so-called complete composite is not obtained in which the rubber material completely penetrates the hollow portion at the center of the cord and the rubber and the cord are integrated. Accordingly, when a composite sheet in which a conventional cord having such a closed-twist structure is held between rubber sheets and subjected to heat compression is combined into, for example, a tire of an automobile, adhesion between the rubber material and the cord is insufficient, and, thus, a so-called separation phenomenon is likely to occur in which the rubber material is separated from the cord during travel of the automobile. Furthermore, when water that has penetrated the rubber material reaches the hollow portion at the center of the cord, the water immediately spreads through the hollow portion in the longitudinal direction of the cord, and causes corrosion of the cord. As a result, a problem occurs in which the mechanical strength of the cord is significantly lowered.
Furthermore, a cord having an open-twist structure is known in which, as shown in FIG. 5, a curling process for intertwining is excessively performed on all of a plurality of wires 31 of a cord 30 having a single-layered twist structure, a gap 32 is formed between the wires 31, and rubber material easily penetrates the interior portion. However, in the case of a cord having such an open-twist structure, the wires are not in contact with each other, and, thus, the shape is easily deformed. Furthermore, the stretch in an extremely low load area (hereinafter, referred to as “low-load stretch”) is large, and, thus, handling work efficiency is poor. Furthermore, due to the tensile force of an extremely low load applied while molding a composite sheet, the gap 32 becomes smaller, and the rubber material may not sufficiently penetrate the interior portion of the cord.
Furthermore, a configuration is proposed in which, as shown in FIG. 6, a wire 41 that is part of a plurality of wires 41 of a cord 40 having such a single-layered twist structure is provided with small spiral curls that are different from curls for intertwining (see JP H5-140882A, for example). This cord is obtained by tightly intertwining a wire that has small spiral curls that are different from curls for intertwining (hereinafter, referred to as a “spiral wire”) and wires that have only curls for intertwining (hereinafter, referred to as “non-spiral wires”). Since the wires are brought into close contact and intertwined into each other, the shape is not deformed as in the case of a cord having a closed-twist structure, and a gap 42 is formed between the spiral wire and the non-spiral wires, so that the rubber material penetrates the interior portion.
However, when a tension load acts on a cord having such a single-layered twist structure in which only parts of the wires are provided with spiral curls in this manner, the tension load tends to be concentrated on the non-spiral wires.
Thus, it is also conceivable that, as shown in FIG. 7, all of wires 51 of a cord 50 having such a single-layered twist structure are formed into spiral wires. In this state, a gap 52 via which rubber material penetrates the interior portion is formed between the wires 51. Furthermore, since the tension load is divided among all of the wires, fatigue resistance is not lowered by the concentration of the tension load. However, when all of the wires 51 are formed into spiral wires in this manner, the low-load stretch of the cord 50 increases.